Support packed sequences in Gated Delta Net

[yapdianang]

Summary

• port [main] feat(moe): Support packed sequence for gated delta net (GDN) NVIDIA/Megatron-LM#2645 onto miles-main
• propagate packed-sequence metadata through GPT and MTP layers
• reset causal convolution and gated-delta state at packed sequence boundaries
• preserve Miles-specific MTP behavior while adding padding-mask propagation

Motivation

Qwen3.8-27B varlen training currently reaches the explicit GDN does not support packed sequence for now guard. Dense Qwen jobs do not exercise this path, which is why the same B300 stack worked for those models.

Testing

• ruff check on all changed files
• Black 24.4.2 check on all changed files
• Python compile check on all changed files
• focused pytest collection attempted; this development host does not have Transformer Engine installed, so GPU test collection stops at that dependency
• B300 end-to-end validation pending a Miles image built from this commit

Upstream implementation: NVIDIA#2645